Dream Machine from Mercedes-Benz

The Sensational C111

Feature Article from Hemmings Sports & Exotic Car

Among car enthusiasts, no question was more pressing: When would Daimler-Benz return to racing? The great Stuttgart company had been front and center in the epic Grand Prix contests of the 1930s and won three World Championships--including that for sports cars--in 1954 and 1955. At the end of the latter year, it withdrew its Mercedes-Benz cars from racing and, for years thereafter, showed no signs of returning to top-line competition.

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Rumors of a new Mercedes-Benz assault on the tracks came as regularly as flowers in springtime. In the mid-1960s, they intensified when the company started buying and testing the new generation of mid-engined cars that had taken over on the tracks. One was a racing Porsche and another was a Lotus Type 47, the racing version of the British firm's Europa coupe. When this was finally broken in during the summer of 1967, its twin-cam engine was wrecked by Daimler executives who couldn't master its dog-clutch gearbox. It lived on for further tests with a dry-sumped BMW 2000 engine.
As early as 1964, a design study for a modern mid-engined Mercedes-Benz sports car was well advanced. However, work on the car was suspended during a stock-taking period that followed the hand-over of Daimler-Benz technical direction from Fritz Nallinger to Hans Scherenberg at the end of 1965. With Scherenberg's appointment, a new engineering generation assumed command at Untertürkheim. Under him, the engineer in charge of automobile development continued to be Rudolf Uhlenhaut, whose intuitive hands-on approach to Mercedes engineering was matched by his outstanding skill behind the wheel.
Uhlenhaut and his colleagues were eager to explore the new world of mid-engined cars with a design of their own. In parallel in the 1960s, another technological advance was demanding attention at Daimler-Benz. On October 26, 1961, the company signed a license to explore the potential of the new rotating combustion engine conceived by Felix Wankel. In part, this was a defensive move; as Rudolf Uhlenhaut said, "The cost of a license keeps going up all the time." Nevertheless, Daimler was the first major company to take the engine seriously; its engineering reputation was such that its purchase of a Wankel license was a huge coup for the radical engine's promoters. Work on the Mercedes Wankel was directed by Wolf-Dieter Bensinger, designer of the engine used in the gull-winged 300SL. Among other engineers on the project were Hans-Otto Derndinger and Heinz Lamm.
With a 450cc chamber volume, the KA-series engine was the first Daimler-Benz Wankel built, over the 1965-'66 winter, specifically for passenger-car installation. KA engines were tested mainly as two-rotor and three-rotor units. Some were evaluated with carburetors, but most of the test-running took place with Robert Bosch fuel injection direct into the intake/compression side of the chamber, where the nozzle is at all times shielded from combustion temperatures and pressures. The adaptation of direct injection to the rotary engine was a distinctive and important contribution to Wankel advancement made by Daimler-Benz.
In 1967, another engine series, the KC, was designed and built, followed in December 1968 by a fourth generation of Untertürkheim Wankels, the KE series. The KE's rotor and housing were widened 5mm, to 75mm, to bring the displacement of a single chamber to 600cc. New also was most of the detail design of the rotor housings and the intermediate sections between the rotors. The triple-rotor version of the KE was called the M950. The total displacement of the three chambers around one rotor assembly was 1,800cc, making the swept volume of the three-rotor engine three times 1,800cc, or 5.4 liters.
As tuned for normal passenger-car use, the M950 developed 210 to 220hp. While respectable, this was not entirely convincing in the production Mercedes cars, which were gradually gaining weight. "In the normal car, it was a little bit underpowered," said development engineer Hans Liebold. "At low revs, it didn't give that much power. Then it was Uhlenhaut's idea to make a little sports car. It suited the engine better. We started with two rotors in the normal passenger car, then three rotors in the sports car."
Thus, two lines of exploration--mid-engined and Wankel-powered--converged to inspire a new car. Early in November 1968, the Daimler-Benz central design office mobilized to start work on a mid-engined coupe, designed around the compact Wankel engine. Engineers K. Enke and E. Löffler were among the designers chiefly concerned with the project, which was given the code name C101.
They designed the car, as they said, "from the driver's seat outward." Starting with the necessary ground clearance, they added the driver's seated height to arrive at an overall height of 44.3 inches. Next, a heavy roll-bar structure behind the seats was established, and behind that, the siting of the Wankel engine with its attached five-speed ZF transaxle. This determined the placement of the rear wheels. The front-wheel distance forward was set by the length needed for comfortable leg distance to the control pedals.
Designed strictly for the three-rotor Wankel engine, the first chassis had a 98.4-inch wheelbase. All later chassis had room for a possible four-rotor engine, provided by a wheelbase of 103.2 inches. Rear track was 54.8 inches. Front track was 54.3 inches with Michelin touring tires and 56.7 inches when equipped with wider wheels and tires that might be suitable for racing.
Several types of frame were evaluated. A multi-tube truss-type frame, drawing on 1952-'55 experience and tradition, was designed and built in scale-model form. However, engineering estimates showed that the car's overall weight would be higher with the tubular frame because it would need an additional inner lining for the passenger compartment. Also, it would give inferior crash protection for the side-mounted fuel tanks and the occupants.
Neither was a full racing-style monocoque frame seen as well suited to a car that was intended chiefly for road use with appropriate interior space and ease of servicing. Thus, the choice fell on the third alternative: a platform frame with deep side sills to provide both torsion and beam stiffness. The first frame was intended to have triangular stiffening webs from its roll-bar arch down to the rearward frame extensions, but, because these would have blocked access to the spark plugs and the exhaust system, diagonal bracing tubes were used instead. Computer studies showed that the rear of the frame was still lacking 18 percent of its attainable stiffness, so it was extensively redesigned for the second and subsequent C101s.
Weighing just over 300 pounds, the deep-sided frame, with its high cowl structure, was fabricated of steel of 0.8mm and 1.0mm thickness. It was not unlike the tub of Ford's GT40, and weighed about the same. Fuel was carried inside the sills flanking the cockpit, with room for 15.8 gallons on each side for a total of 31.6 gallons. Built for crash-test evaluation were several types of internal tanks. One was a rubber bladder, as used in competition cars, while another was an aluminum insert with a corrugated skin designed to deform progressively and predictably in a collision.
Brakes, usually challenging to construct for a high-performance sports car, were readily available for the C101, borrowed from the 300 SEL 6.3 sedan. In touring form, a brake servo was used, but none was fitted to cars that were tuned for track use.
A production Mercedes-Benz also provided a recirculating-ball steering gearbox, but the rest of the C101 front end was novel. It was designed around the concept that the front-wheel steering axis (the line formed by the two steering ball joints) should intersect with the surface of the pavement exactly at the center of the tire contact patch. By achieving this "zero offset," the designers intended to gain maximum stability under heavy braking. Conventional designs offset this axis/road intersection by 10 to 25 percent of the tire tread width from the center of the contact patch, making steering easier at low speeds.
It wasn't easy to get zero offset. Enke and Löffler did it by recessing the lower ball joint deeply into the center of the brake disc, at the back of the spindle. A forged-steel arm formed the main part of the lower guidance member, triangulated by a bracing rod to the rear. From the spindle, another forged arm curved upward, comprising the steering knuckle, to a ball joint located at the height of the center of the tire section. The very high placement of this upper ball joint permitted the top suspension links to be relatively short. This saved installation and luggage space without causing exaggerated wheel caster and camber. Front-wheel travel was accommodated by a tall coil/shock assembly extending high up into the front frame members. Linked to the main lower suspension arm, close to the shock mount, was a massive front anti-roll bar.
At the rear, an initial attempt was made to use off-the-shelf suspension parts: the angled trailing arms that were introduced on the smaller W107 sedan models in 1968. The first short-wheelbase chassis was so equipped when it rolled onto the test track at Untertürkheim, first as a bare chassis and later with a crude body that the engineers called the "Tin Box," early in April 1969.
The car's first trip on the open road, on April 16, was shared by Rudolf Uhlenhaut and a 42-year-old engineer who was also a skilled driver, Hans Liebold. Named C101 project manager, Liebold spent much of his working life involved with this remarkable car. He took it on its first runs at the Nürburgring and Hockenheim in May 1969. First impressions, said development engineer Fritz Naumann, weren't good: "Weak performance, poor handling, miserable shifting. But that was only a beginning."
Handling was less bad with touring tires on the rear semi-swing axles. But with wider racing tires, the suspension's drastic camber changes caused excessive rear-wheel steering. Enke and Löffler went back to their drawing boards; No fewer than 300 different linkage combinations were computer-plotted to pinpoint the one that would do the job best.
The engineers suspected that some of the instability problems experienced with the first Tin Box body had aerodynamic origins. They put it in the new Daimler-Benz full-scale wind tunnel and found that its high fenders and flanks were not helping. Trimming them with tin shears along pen-marked lines allowed the tests to continue. Something more professional in the way of bodywork was meanwhile being prepared by the body engineering group under Werner Breitschwerdt and Karl Wilfert. "Our objective," recalled Wilfert, "was a functional car with low aerodynamic drag. It was to be without lift and without movable flaps or spoilers."
A simplified model of the car went into the Daimler-Benz wind tunnel at Sindelfingen at an early stage. Embodied in it were two internal air-flow features: large scoops at the rear for engine oil coolers and a water radiator set into the front deck so that air entered it through a hole near the front of the upper deck surface and departed from it through a grille at the base of the windscreen.
These airflow requirements strongly influenced the body design. So did the front suspension, with its tall knuckle and coil/shock unit. It called for extra clearance over the front wheelhouses that forced the cowl line high and led the stylists toward a high beltline and a tapered wedge shape. Inputs from the tunnel tests found their way back to the styling studio, where scale-model refinements were applied to a full-size clay model. By the end of January 1969, after only two months, the styling was completed.
In making the body, Breitschwerdt and Wilfert decided to explore a technique relatively new to Daimler-Benz: glass-fiber-reinforced resin, otherwise known as fiberglass. The bodies were built by the Rastätter Waggonfabrik, a firm west of Stuttgart located near the big Daimler-Benz truck plant at Gaggenau. Normally producers of fiberglass truck bodies, they were asked to adapt their techniques to the C101.
The full-sized clay model on its wooden armature was trucked west to Rastatt to begin the body-making process. Female fiberglass molds were formed against the clay model, which was destroyed when they were removed. In these molds a heavy body shell was laid, an "intermediate body" that was refined by hand. The intermediate body was painted and equipped with wipers, lamps and license plates for the approval of Daimler-Benz management. It then served as the male master for the final set of female body molds. From these, the fiberglass skins were pulled. They were made 2mm to 3mm thick in a single piece from the nose through the roof spine to the sides of the rear fenders.
On July 15, 1969, a C101 was ready to take to the road with its final body glued, riveted and screwed into place. Tufted for visual flow observation, it was taken to the veteran F.K.F.S. wind tunnel in Stuttgart to see how well its shape worked. Drag was satisfactorily low, with a coefficient of 0.335 and a frontal area of 17.9 square feet. Contributing to its low drag was the way its windows were glued in place, flush with the body surface. Stability was good too. At 155 mph, the C101 experienced a 66-pound downforce on the front wheels and a lift of only 22 pounds at the rear. The Sindelfingen stylists had admirably achieved their design objective with nary a spoiler in sight.
In the meantime, road tests at Untertürkheim and at Hockenheim with the Tin Box, which had by then acquired a rough fiberglass nose, showed unexpected problems. The oil coolers, located in the rear, weren't getting enough air. Hot to begin with, the engine compartment was now unbearably so. Also, the system of exhausting the front radiator air at the base of the windshield was making the glass hot, which, in turn, heated up the interior. Major revisions were needed.
The spare tire, placed flat in the nose of the first car, was moved back and angled beneath the decklid, to make room at the very front for a radiator with an oil cooler ahead of it. Air was now taken in through a black-painted inlet in the nose, while the slatted grille in the front deck that had formerly been the air inlet became the outlet instead. The front end was lengthened several inches to make room for this. At the same time, the hidden-headlamp operation was converted from a slide-up action to a tilt-up design, still worked by a cockpit lever.
Replacing the squared-off instrument board of the Tin Box was a simple free-form leatherette-trimmed fiberglass dash with minor instruments in its center, angled toward the driver. Seats and door inner surfaces were upholstered in the tartan cloth that had been a tradition since the racing 300SLs of 1952. Fixed side windows had lever-operated vent flaps, and a knob between the seats allowed the narrow rear window to be slid open or closed. First seen on the C111 was a new-design four-spoke steering wheel with a basket-weave-surfaced rim and horn-push crescents flanking a flat center; it was destined for future production-car use. With the tub's wide sills, it was logical to give the C111 the same gull-winged doors that had distinguished the 300SL.
Adequate though it had been in the 1950s for the 300SL, the mere 220hp of the sedan version of the triple-rotor engine wouldn't have been enough to propel the 2,420 pounds of the C111 in the sporting style of the late 1960s. Accordingly, it was hot-rodded in the best Stuttgart style, its output soaring to 330hp gross and 270 to 280hp at 7,000 rpm, with accessories, as installed. It was by far the most potent Wankel engine hitherto installed in a car.
On July 5, 1969, the first chassis tub was delivered to Rastatt to be fitted with its body. More tubs followed in regular succession, then returned to Untertürkheim to have their suspensions and engines fitted. All thoughts of holidays went by the boards, for the Daimler-Benz management had decided to show the exciting new car at Frankfurt's IAA, the international auto show, in September. "We worked through the nights," said Fritz Naumann, "the mechanics in shifts, Liebold and I with our own timetable. I put a cot in my office so I could snatch a few minutes' sleep from time to time. In one month I recorded 165 hours of overtime. Whether it was paid I cared not; I was totally besotted with the C101." Because Peugeot had protected three-digit-type designations using a 0 in the middle, Daimler-Benz management board decided on August 5, 1969, to call it the C111.
Even in this early form, scarcely out of the womb, the C111 was quite a car. With its ZF differential giving a 70 percent slip limitation and its 55 percent rearward weight bias, the 1969 C111 would spin its rear wheels from a racing start. Moving up the speed range, the coupe reached 55 mph in first, 90 in second, 115 in third and 140 in the fourth of the five speeds of its ZF 5DS-25 transaxle. The 1969 C111s were geared to reach 165 mph at 7,000 rpm in fifth, which they could attain given enough distance.
Notorious for its poor shift quality, the ZF gearbox was given a new and superior shift linkage by Fritz Naumann. When the test drivers had trouble getting smooth downshifts, such was the engine's eager response to the throttle, one of Liebold's engineers had a solution. With a simple linkage, he added a supplementary throttle pedal, low and to the left, that had a lower operating ratio than the main pedal. With this novel two-speed accelerator, quick and smooth downshifts were possible with less danger of over-revving.
Demonstrations and test drives at Hockenheim of three cars on September 1-5 revealed that the C111's three-rotor Wankel spread its power over a broad range, idling smoothly at about 1,000 rpm and accepting full throttle without stumbling as low as 1,600 rpm. From there, the power climbed with authority, thrusting the C111 forward with a sense of unleashed energy as its torque leveled off between 5,000 and 6,500 rpm before peak output was reached at 7,000. At 7,500 rpm, a governor signaled the injection pump to cut down the fuel supply.
Excruciatingly noisy in the test cell--"well beyond human endurance," said one engineer--the Wankel in the C111 was heavily muffled. From the outside, it had a gruff, deep note as background to a metallic ringing that recalled a two-stroke at speed. Although not quiet inside the C111, its sound was adequately suppressed. It resembled a whirring or singing like that of a huge electric motor.
That the goals of designers Enke and Löffler for the chassis of the C111 were admirably achieved was evident at the Hockenheim demonstrations. In "tuned" form, the C111 generated a cornering acceleration of 1.5G on Untertürkheim's 300-foot skid pad. "It's not so easy to achieve, you know," said the half-English Uhlenhaut with typical understatement, "and it's impossible to use on the highway."
At Hockenheim, the engineer demonstrated how much could be used. Uhlenhaut hurled the "tuned" C111 through its fast bends at rates that glued his passenger against the side of his seat. The precise yet supple suspension allowed stunningly fast cornering with precisely neutral handling on both fast and slow turns. The driver had only to aim the C111 and stay on the throttle until he heard mutterings from the tires that signaled that they were starting to work hard. Compromises were more evident in the "touring" version. While on the same Stuttgart test circle, it generated 0.9G in cornering--unsurpassed by any other road car at that time and indeed matched by few today--the lack of a rear anti-roll bar caused the car to understeer strongly.
"There is no doubt that this is a very fast road car," summed up racing driver Paul Frère after a C111 drive in the Black Forest, "and, what is more important, a car which can be driven consistently fast, for it is utterly safe and very restful. In this, it completely fulfills its aim, which is not to be a racing car but a worthy successor to the immortal gull-winged 300SL, the fastest practical road car of its time."
Those who tried the C111 that September 1969 had only two major complaints. One was that the engine, tuned for peak power, was lacking in torque below 4,500 rpm. The other was that its high cowl and narrow windows weren't ideal for visibility. Daimler-Benz was readying solutions to both these problems, solutions that would be seen--surprisingly soon--in 1970. There was a lot more to come in the story of the C111. It might not have been the car that would bring Mercedes-Benz back to racing, but it was shaping up to have the potential to be a sensational sports car for the road, potentially by far the best of the new mid-engined generation.

This article originally appeared in the November, 2005 issue of Hemmings Sports & Exotic Car.